1. Field of the Invention
This invention relates to a sewage pump assembly, and more particularly to a sewage pump assembly which is sunk into filthy water contained in a cesspool constructed, for example, in the lowest basement of a building, and by which the filthy water is raised to a sewer or the like.
2. Description of the Prior Art
As well known, a cesspool is constructed in the lowest basement of a building. Waste water or excreta from toilets of the building, slops from different parts of the building and the like are led into the cesspool, and they are temporarily stored there.
The sewage pump assembly is equipped with an automatic drive apparatus. When the filthy water is stored up to a predetermined level in the cesspool, the sewage pump assembly is automatically operated with the automatic drive apparatus, and the filthy water is raised up to the sewer from the cesspool.
The filthy water containing foul materials is apt to rot. The rot filthy water give out a very bad smell. When it flows through the sewer, the very bad smell is given out from a manhole in a street. That is a kind of public nuisance.
One of causes for discharging the rot filthy water is that the frequency of pumping operations is very low. However, the most important cause is that different floating materials on the filthy water such as oil, dust, mosquito, larvae and maggots increase and will facilitate the rotting of filthy water in the cesspool.
Accordingly, it is required for any sewage pump to discharge the floating materials on the filthy water as much as possible.
For such a purpose, a sewage pump assembly as shown in FIG. 1 has been disclosed by this applicant in the Japanese Utility Model Publication No. 48722/1976. Referring to FIG. 1, a long vertical suction pipe 43 is connected to an inlet port 42 of a sewage pump 41. An upper inlet opening 44 directed upwardly and positioned at a predetermined level relative to the sewage pump 41, and a lower inlet opening 45 positioned near the bottom of a cesspool 46 are connected through curved pipes to the long vertical suction pipe 43. When the pump 41 is driven, some of the floating materials and settlings are sucked through the upper and lower inlet openings 44 and 45 into the pump 41.
However, the floating materials are sucked only for a very short time before the pump 41 is stopped. The discharging amount of the floating materials is thus limited.
The sewage pump 41 is controlled with an automatic drive apparatus. When the water level of the filthy water has reached a predetermined upper limit level L.sub.1 in the cesspool 46, the drive of the sewage pump 41 is automatically started by upper level control 60a to suck the filthy water through the upper and lower inlet openings 44 and 45 and to discharge it outwards through a discharge pipe 47. And when the water level of the filthy water has reached a predetermined lower level L.sub.2, the drive of the sewage pump 41 is automatically stopped by lower level control 60b. The draw of the filthy water is stopped.
The pump 41 is automatically stopped at the lower limit level L.sub.2 which is higher by 1 to 3 cm than the upper inlet opening 44. Thus, there is some clearance between the level of the upper inlet opening 44 and the lower limit level L.sub.2. Accordingly, the pump 41 is prevented from sucking air to run idle and to waste electric power.
At the initial stage of the pumping operation when the water level of the filthy water is nearer to the predetermined upper limit level L.sub.1, the floating materials on the water level cannot be sucked into the upper inlet opening 44, since the upper limit level L.sub.1 is further higher than the upper inlet opening 44. The water level lowers with the pumping operation. When the water level of the filthy water has reached such a level as nearer to the lower limit level L.sub.2, for example as higher by 5 to 6 cm than the upper inlet opening 44, the floating materials on the water level start to be sucked into the upper inlet opening 44. However, after that, the water level of the filthy water reaches the lower limit level L.sub.2 in a short time. The sewage pump 41 stops.
The time for which the floating materials are discharged through the upper inlet opening 44 by the pump 41, is very short. The discharging amount of the floating materials such as oil is very little. Moreover, when the pump 41 is stopped, the filthy water in the discharge pipe 47 is returned through the upper and lower inlet openings 44 and 45 into the cesspool 46 with its dead load. Also the floating materials are returned into the cesspool 46 together with the filthy water. As the result, the discharging efficiency of the floating materials is reduced.